When directly printing onto containers, such as bottles, a print advance rate of the surface to be printed on relative to at least one ink-jet print module is achieved in that the container is in the region of the print module rotated about itself and/or passed by the print module along a predetermined transport path. A plurality of partial prints is, after setting a suitable rotational orientation of the container and while maintaining a print advance rate as constant as possible, then combined at the respectively associated print heads or nozzle rows to create one print image in a direct printing process.
To print onto containers at a known rotational velocity and at a known rotational orientation about their main axis, it is known from WO 2010/108527 A1 to position the containers to be printed onto turntables, where line markings or the like are applied at the circumference of the turntable with even spacing for monitoring the rotational orientation of the container. The latter can thereby be adjusted relatively accurately in front of print heads.
The problem remains, however, that the dimensional and/or shape tolerances of the container, for example, unwanted eccentricity of the container cross-section, cause print advance rates of different velocities of the surface to be printed on in front of the associated print heads or nozzle rows when rotating the container about itself or when moving it along curved transport paths. These fluctuations in the effective local print advance rate in the case of conventional nozzle control have the effect that the resolution of the ink-jet print, i.e. the spacing between each ink droplet, varies along the container circumference. Moreover, connecting regions with overlapping prints or with gaps arise when assembling the partial print images that have been created by use of different print heads or nozzle rows.
Due to the manufacturing process, in particular glass bottles exhibit relatively large tolerances in dimensions and shapes. When rotating glass bottles with a rotationally-symmetrical nominal cross-section, for example, the container wall laterally impacts due to its eccentricity, which has previously prevented commercial use of ink-jet direct printing onto glass bottles.
Similar problems exist for ink-jet direct printing onto shaped bottles that are by definition not rotationally-symmetrical. Though it is known from EP 2 459 385 B1 to adapt the position and orientation of ink-jet print heads to the contour of shaped bottles to be printed on, there is nevertheless the aforementioned problem also for shaped bottles of varying print resolution and/or excessive overlap and/or incomplete assembly of partial prints due to the varying radii of the trajectories of individual circumferential portions of the container sidewall.
Due to the high print resolution commonly demanded, print heads may be used that have a plurality of nozzle rows. Such nozzle rows or nozzle blocks have defined offsets relative to each other. If a predetermined print advance rate is departed from, then undesirable distortions in resolution of the pixels and double prints arise.
A need for methods and devices for ink-jet printing onto containers therefore exists in which at least one of the above-mentioned problems is eliminated or at least alleviated.
This object posed is satisfied by a method according to claim 1. This method therefore serves ink-jet printing onto containers, where a print advance rate in front of at least one print module is achieved at least by rotating the containers about themselves and/or by transporting the containers along at least one curved trajectory, in particular by circulating them on a carousel. The surface velocities of lateral portions of the containers are there measured during the rotation and/or the transport. Furthermore, time intervals between printing times of the print module and/or an angular velocity of the rotation of the containers about themselves are set in dependence of the measured surface velocities. The latter then correspond to actual print advance rates of individual lateral portions of the containers relative to the print module.
The measured surface velocities can be caused solely by rotation of the containers about themselves, i.e. by rotation about an axis that is stationary relative to the print module, or by superimposing the container rotation about itself a transport motion of the containers, i.e. by rotating the containers about an axis of rotation that moves relative to the print module, for example, along a linear transport path or along a curved transport path. Both linear conveyors as well as carousels or otherwise curved conveying stretches are suitable for this.
The measured surface velocities can likewise be caused solely by circulating the containers in a carousel or by a motion along otherwise curved transport paths. The rotational orientation of the containers about themselves is then respectively adjusted upstream of the velocity measurement according to the invention.
For example, it is possible that the containers are on a carousel or a similar transport device and pass along the print module and are there during printing also rotated about themselves. Print advance is then achieved by superimposition of the transport motion and rotating the container about itself. In all the cases described above, the surface velocity measured according to the invention is representative of the actual print advance rate of each sampled lateral portion of the container surface.
The lateral portions are, for example, partially circumferential portions of a side wall to be printed on/or representative of its circumferential line. The lateral portions can directly adjoin one another, for example, when continuously sampling the surface along the container circumference. The lateral portions can also be spaced apart, within the meaning of a measurement point grid pattern extending along the container circumference. Printing times and associated time intervals can be calculated for lateral portions between the measuring points of the grid pattern, for example, by interpolation of readings. The lateral container surface is optionally sampled from a position which is stationary relative to the print module.
By adapting the printing times and/or the angular velocity of the container rotation about itself, deviations of the actual print advance rate of individual lateral and/or intermediate portions of the container from the target print advance rate can be compensated with respect to at least one print head and/or with respect to a nozzle row oriented in particular transverse to the advance direction, in order to produce a print resolution in the advance direction that is as uniform as possible.
By adapting the angular velocity, i.e. the rotational velocity of the containers about themselves, deviations of the reference advance rate of individual lateral and/or intermediate portions of the container from a target advance rate can be compensated with respect to at least one print head and/or with respect to a nozzle row oriented in particular transverse to the advance direction, in order to produce a print resolution in the advance direction that is as uniform as possible. For example, the angular velocity/rotational velocity is for a measured deviation from a target value of the angular velocity/rotational velocity and thereby from the print advance rate readjusted in order to keep the deviation within an allowable tolerance range.
For this purpose, for example, courses of the angular velocity/rotational velocity can be created for an entire or partial circumferential rotation of the container about itself and possibly be stored in order to reproducibly alter the angular velocity/rotational velocity in front of different printheads such that a respective substantially constant print advance rate of the surface to be printed on arises in front thereof.
This would be conceivable, for example, for carousels at which a specific color is printed or a specific treatment step is performed. The individual container or container type can then be assigned an individual course or rotational velocity that the container maintains for the individual print modules or pre-/post-treatment modules on its way through the device according to the invention, for example, through multiple carousels.
Time intervals being associated with the different lateral portions and/or intermediate portions may be set respectively greater, the lower the associated surface velocities. Adapting the printing times is therefore to be understood such that, for lateral portions with a relatively high surface velocity, print commands for a print head for a nozzle row oriented transversely to the advance direction and/or for a single nozzle are given having comparatively short time lags to each other, and for lateral portions with comparatively low surface velocity in contrast with larger time lags. This allows an actual print advance rate of the container surface of different velocities along the container circumference to be compensated in order to place ink droplets thereon at a spacing in the advance direction as uniform to each other as possible.
The time intervals between printing times of individual nozzles and/or nozzle rows of the print module may be defined, in particular between immediately successive printing times. The adapted time intervals are associated with the lateral portions of the container surface and can thereby be applied to nozzles and/or nozzle rows of different print heads or print modules in order to adapt ejection of ink to the respective actual print advance rate. Unwanted print artifacts at the transition between partial prints produced with different nozzle rows, printheads and/or print modules, for example, an overlapping print or connection gaps, can thereby be suppressed.
The surface velocities may be measured during ongoing print advance, in particular during ink-jet printing. This is to be understood such that the motion responsible for print advance is from measuring the surface velocities to the associated print process not interrupted. The rotational position of the container then does not necessarily need to be determined for the adaptation of printing times according to the invention. The printing times can instead be adapted essentially on-the-fly, for example, during rotation at a constant angular velocity and in consideration of a time offset until the respective nozzle or nozzle row is reached. This is advantageous in particular for glass bottles, for which the individual dimensional and shape tolerances are in the foreground, so that printing times for every bottle must be corrected individually.
The surface velocities may be measured at a known angular velocity during rotation and/or transportation. The known angular velocity is optionally constant, but can be varied, provided that the measured surface velocity of the angular velocity there applied can be allocated. The angular velocity can further also be re-adjusted or controlled in order to reduce or compensate any deviation of the measured actual print advance rate from a target print advance rate. This may be done on-the-fly or in the form of a previously stored course of the angular velocity. The known angular velocity can be overlaid by a transport velocity likewise known, for example, along a linear conveyor section.
Alternatively or additionally, the measured surface velocities are each associated with measured rotational positions of the container. Readings can be, for example, stored together and also be used for subsequent print processes to calculate printing times and/or adapted courses of the angular velocity. The fluctuations of the surface velocity of individual lateral portions, caused by eccentrically held and/or non-rotationally-symmetrical container cross-sections, could in principle also be measured and stored in an upstream method step. Time intervals between printing times and/or angular velocities associated with individual rotational positions of the container can subsequently be repeatedly used for any number of printing processes of the same lateral portions. This is advantageous for shaped bottles made of plastic, whose deviation from a rotationally-symmetrical cross-section is defined and which have minor individual dimensional and shape tolerances as compared to glass bottles.
The surface velocities may be measured using a friction wheel rolling laterally along the container, a functionally equivalent roller or the like. Coupled thereto is, for example, a rotary encoder for accurate digital velocity measurement. The friction wheel can be adjusted, for example, in the vertical direction to sample the container sidewall on a height level that is representative of the wall contour to be printed on. The friction wheel then may roll along the entire circumference of the container. Friction wheels are particularly suitable for bottles with rotationally-symmetrical nominal cross-section.
Alternatively or additionally, the surface velocities can be measured in a contactless manner by optical scanning of the lateral portions and/or by acoustic sensing by way of ultrasound. This is particularly advantageous for large relative velocities between the container surface to be measured and the measuring device and/or a short residence time of the container in the region of the measuring device/the print module.
The printing times and/or the angular velocity may also be adapted to print distances from the lateral portions of the containers and/or intermediately disposed portions. Run-time differences of individual ink droplets from the nozzles to the portions of the container surface to be printed on can thereby be compensated.
The containers are optionally glass bottles, in particular such with rotationally-symmetrical nominal cross-section, or shaped bottles, in particular such made of plastic. Glass bottles have particularly high dimensional and shape tolerances due to their fabrication method, in particular in terms of their outer circumference and their eccentricity toward the bottle mouth. Compensation of different actual print advance rates of individual sidewall portions by adapting the associated printing times is therefore particularly important for glass bottles or even a prerequisite in terms of acceptable quality when using direct printing by way of ink-jet.
Due to the non-rotationally-symmetrical nominal cross-section, shaped bottles inevitable and in a possibly particularly pronounced manner exhibit different actual print advance rates of individual sidewall portions both during rotation about themselves as well as during transport along curved trajectories, respectively, following a rotation about themselves. Printing times adapted according to the invention even enable prints with uniform print resolution in the advance direction on container cross-sections having a complex shape.
The object is also satisfied by a device according to claim 11. According thereto, it is used for ink-jet printing onto containers and comprises: at least one print module; at least one positioning unit for holding and rotating a container about itself in front of the print module; at least one measuring device for determining surface velocities of lateral portions of the rotating container; and a control device for actuating the print module while adapting time intervals between printing times of the print module in dependence of the measured surface velocities. The device is then, for example, a cyclically operated device of the stationary type in which the containers do not circulate in a carousel, or a device of the rotary type, in which the print modules circulate together with the containers. It is also conceivable that the containers held by the positioning unit continuously pass by along the at least one print module, for example, along a transport stretch extending linearly in the region of the print module.
The object is also satisfied by a device according to claim 12. According thereto, it is used for ink-jet printing onto containers and comprises: at least one print module; a carousel with positioning units circulating therein for holding and rotating the container about themselves; at least one measuring device for determining the surface velocities of lateral portions of the circulating containers; and a control device for actuating the print module while adapting time intervals between printing times of the print module in dependence of the measured surface velocities.
The containers can be rotated both in front of stationary print modules to create a print advance, as well as in front of circulating print modules. For example, the print modules could each circulate on carousels through which the containers pass in series, where the carousels then may be each assigned a particular color of a color model or perform a specific pretreatment step/post-treatment step, such as curing.
Carousels being assigned a certain partial print step or treatment step can be modularly inserted in the serial sequence of carousels or removed therefrom according to the required color and/or processing steps. The succession of carousels could be added inlet modules and outlet modules. The containers could for printing further be inserted into transport adaptors or other transport/positioning aids.
Measurement of the surface velocity according to the invention can be applied selectively for the correction of printing times and/or the adaptation of the angular velocity/rotational velocity of the containers for printing individual circumferential partial portions using a specific print head.
Printheads and units for curing print could also be formed in a common horizontal plane, in particular in star configuration around a positioning unit for holding and rotating a container about itself. Measurement of the surface velocity according to the invention can then be used for correcting printing times at the printhead or the like which is currently facing the measured surface.
Adaptation of printing times/rotational velocities according to the invention could likewise be used for print modules in which the printheads are arranged one above the other, i.e. the containers are for partial print change/print head change driven along their longitudinal axis and may be printed in different horizontal planes.
The measuring device may comprise a friction wheel with a rotary encoder, where the friction wheel is resiliently preloaded in the direction of the container to be sampled. The friction wheel can in a simple manner be coupled directly to the print module.
The printhead and the friction wheel are supported or optionally mounted jointly movable in the direction of the container. When the friction wheel then rolls along the container, a constant print distance between the container surface and the nozzles/nozzle rows of the print module then arises. The friction wheel then acts as a guide roller for the nozzles/nozzle rows. The container surface then acts as a corresponding guide curve.
The measuring device may operate in a contact-less manner on the basis of an optical and/or acoustic scanning beam. Scanning is then performed, for example, by way of laser light or ultrasound. Optical code readers, line scanners, cameras or the like are suitable for optical scanning.